1. Field of the Invention
The present invention relates to a sound system for a vehicle, and more particularly, to a sound system capable of increasing and decreasing the volume value of a reproduced audio signal in accordance with the level of road noise caused by the movement of a vehicle.
2. Description of the Related Art
In a vehicle sound system, since reproduced sound is masked by the influence of noise from the engine or what is called road noise arising between tires and the surface of a road, it is necessary to adjust the audio volume of the sound system in accordance with the level of such noise.
Conventionally, the reproduced sound is prevented from being masked by detecting road noise by means of a microphone or the like and controlling the electronic volume so as to obtain a volume value corresponding to the road noise. FIG. 5 shows the relationship between the noise value and the volume value. Referring to FIG. 5, the horizontal axis and the vertical axis indicate the road noise value and the volume value, respectively. The volume value is controlled based on this relationship. For example, if the reproduced audio volume is -40 dB (point a), when the value of the road noise is increased to 3 V, the audio volume is also increased to the volume corresponding to the noise value of 3 V, for example, -20 dB (point b).
Since the volume control characteristic shown in FIG. 5 is normally set previous to the shipment of a vehicle sound system, it cannot be changed after the vehicle sound system is mounted on a vehicle. In other words, since the degree of influence of road noise on reproduced sound varies according to the vehicle, when the volume control characteristic is predetermined, even if the volume value is increased by the same amount in response to the increase of noise, the reproduced sound is still masked in some vehicles while the volume value is too high and the reproduced sound level is too high in other vehicles. Therefore, it is not practically possible to determine the volume control characteristic at the shipment of a vehicle sound system.